Document NEZGLBBKm83B8RBBpe9J4wgkE
HEATINC VENTILATINC AIR CONDITIONING GUIDE 1944
is considered good practice to allow 20 per cent additional for weights of joints and bracings. Various weights and thicknesses of standard copper sheets will be found in Table 7.
Table 6. Weights of Sheet Metal Used for Duct Construction
u. s.
Std.
Gage
Black Sheets
Approximate Thickness, In.
Weight Per Square Foot
Galvanized Sheets
Approximate Thickness, In.
Weight Per Square Foot
Steel
Iron
Ounces Pounds
Steel
Iron
Ounces' Pounds
30 28 26
24 22 20 18
16 14
12
n
10
0.0123 0.0153 0.0184
0.0245 0.0306 0.0368 0.0490
0.0613 0.0766 0.1072 0.122q 0.1379
0.0125 0.0156 0.0188
0.0250 0.0313 0.0375 0.0500
0.0625 0.0781 0.1094 0.1250 0.1406
8 10 12
16 20 24 32
40 50 70 80 90
0.500 0.625 0.750
1.000 1.250 1.500 2.000
2.500 3.125 4.375 5.000 5.625
0.0163 0.0193 0.0224
0.0285 0.0346 0.0408 0.0530
0.0653 0.0806 0.1112 0.1265 0.1419
0.0165 0.0196 0.0228
0.0290 0.0353 0.0415 0.0540
0.0665 0.0821 0.1134 0.1290 0.1446
10.5 12.5 14.5
18.5 22.5 26.5 34.5
42.5 52.5 72.5 82.5 92.5
0.656 0.781 0.906 '
1.156 1.406 1.656 2.156
2.656 3.281 4.531 5.156 5.781
"Galvanized sheets are gaged before galvanizing and are therefore approximately 0.004 in. thicker.
Table 7. Weights and Thicknesses of Standard Copper Sheets'5 Rolled to Weight
Weight per Square Foot
. Thickness, Inches
Ounces
Pounds
. r Decimal Equivalent
Nearest Fraction
10
0.625
0.0135
Hi
12
0.750
0.0162
14
0.875
0.0189
Hi
16
. lb
20
24
1.000
1.125 1.250 1.500
0.0216 0.0243 O.027O 0.0324
%2
M2 M2 H2
28
1.750
0.0378
32
2.000
0.0432
Hi
36
2.250
0.0486
40
2.500
0.0540
Hi
44
2.750
0.0594
Me
48 56
3.000 3.500
0.0648 0.0756
Me
Hi
64
4:000
0.0864
Hi
^Variations from these weights must be expected in practice.
Nearest Gage No.
B. &S.
Stubs
U. S. Std.
27 29 29 26 27 28 25 26 26
23 24 25 22 23 - 24 21 22 23 20 21 22
19 20 20 17 19 19 16 18 18 15 17 17
15 17 14 16
13 -'15
11 14
17 16 14 13
HEAT LOSSES FROM DUCTS The thermal transmission coefficient U for an uninsulated metal duct can be obtained from the equation:
V=
-L+-L
fi So
614
(8)
CHAPTER 32. AIR DUCT DESIGN
In the case of non-metallic ducts the formula in Equation 8 will become: (9)
where
U = thermal transmittance, Btu per square foot per hour per degree Fahrenheit difference in temperature between the average temperature inside the duct and the air outside the duct.
fi -- film conductance inside the duct, Btu per hour per square foot per degree Fahrenheit.
/o> -- film conductance outside the duct, Btu per hour per square foot per degree Fahrenheit.
x = thickness of duct wall in inches.
k = conductivity of duct material, Btu per square foot per hour per degree Fahren
heit difference between the two surfaces of material.
Where * is small and k is large, however, this factor is of little im
portance and may be neglected.
Film conductance/i for air flowing in ducts apparently depends only on the velocity of the air and the diameter of the duct. A fairly reliable inside coefficient can be calculated from Schultz's modified equation:
, 0.32 To0-* j)ojs
where
. Vo = velocity of air in duct, feet per second.
D = diameter of duct, feet.
(10)
Film conductance/o depends on a number of variables including tem perature, diameter, and emissivity of the outer surface and can readily be calculated from data in Chapter 3. From this explanation, it is seen that it is unwise to recommend a given value of U for all uninsulated metal ducts.
' The heat loss from a given length of duct can be expressed by:
Q = UPL
- < ]
(H)
The heat given up by the air in the duct is:
Q = 0.24 M (/, - <,) = 14.4 A Vp (/, - *,)
(12)
Equating 11 and 12 enables the determination of the temperature drop
in the duct:
tj 4: h -- %
28.8 AVp
<i - h
UPL
Let y =
UJr-Lt
solving for h and h :
fr rectangular ducts, = ^
U-Lu
<. (y + 1) -- 2f,
Ii =
b - l)
h (y -- 1) 4- 2f
(y + l).
615
for round ducts,
(
(13)
(14)
nji i i' i 11
i